This invention relates generally to a device for detecting a change in internal pressure of a cylinder and, more specifically, to a mounting structure of a piezoelectric sensor mounted to a cylinder head of an internal combustion engine to detect a change in pressure within the cylinder caused by knocking or fuel injection. The present invention is also directed to a method of the manufacture of a piezoelectric sensor.
One known internal pressure detecting device is illustrated in FIGS. 25 and 26. Designated as S is an annular piezoelectric sensor composed of a metal shell surrounding a laminate including an annular piezoelectric element and upper and lower cushioning rings. The sensor S is fixed on a plug seat r.sub.2 by tightening with an ignition plug T so that the pressure change within the cylinder is transmitted to the sensor S as a mechanical strain to cause the sensor S to output a corresponding electrical. More particularly, the cylinder head P is provided with a plug insertion hole r.sub.1 having the plug seat r.sub.2 and an internally threaded portion r.sub.3. The ignition plug T has a stepped portion t.sub.2 engageable with the plug seat r.sub.2 and an externally threaded tip end portion threadingly engageable with the internally threaded portion r.sub.3, so that when the plug T is screwed in the plug hole r.sub.1, the sensor S disposed on the plug seat r.sub.2 optionally through a gasket, is tightly fixed thereon. The shell 4 of the sensor S is composed of an outer ring plate 5a, an inner ring plate 5b and an annular bottom plate 5c by which an annular space 6 is defined. Disposed in the space 6 are a pair of annular piezoelectric elements 1 and 1, an annular electrode plate 2 interposed between the piezoelectric elements 1 and 1, and a top cushioning ring 3. These parts are coaxially aligned and integrally united with an adhesive or by caulking. The electrode ring 2 has a portion 2a which extends radially outwardly through the outer ring plate 5a and to which a lead wire 7 is electrically connected to send an electrical signal from the sensor S to a suitable signal detecting device (not shown).
In the above mounting structure, the sensor S is pressed between the plug seat r.sub.2 of the plug insertion hole r.sub.1 and the stepped portion t.sub.2 of the plug T. Thus, the tightening torque of the plug T is acted on the sensor S directly or indirectly through a gasket. Since such a torque includes both axial compression stress and tortional or twisting stress, the sensor S which has relatively low shearing strength tends to be broken or damaged and is deteriorated in the piezoelectric characteristics thereof. To cope with this problem, a stopper is disposed for preventing the rotation of the sensor S at the time of the mounting on the cylinder head P. The use of such a stopper, however, is disadvantageous from the standpoint of economy and productivity because the mounting structure becomes unavoidably complicated. Further, the use of a stopper results in the increase in distance between the top surface of the sensor S and the plug seat r.sub.2 so that it becomes difficult to locate the tip end of the plug P, at which spark discharge takes place, at an appropriate position.